Various implantable medical devices are available for use in monitoring various physiological parameters. For example, U.S. Pat. No. 4,360,030 to Citron et al., entitled, "Apparatus For Monitoring And Storing A Variety Of Heart Activity Signals," issued Nov. 23, 1982, describes a heart monitoring and storing apparatus for evaluating heart activity signals. Further, for example, U.S. Pat. No. 5,535,752 to Halperin et al., entitled, "Implantable Capacitive Absolute Pressure And Temperature Monitor System," issued Jul. 16, 1996, describes a monitor that powers a sensor and which demodulates and stores absolute pressure and temperature data derived from signals generated by the sensor. Generally, an implantable device used for monitoring receives sensor output signals from one or more sensors, and monitors, records, and stores data representative of such signals when the device is implanted in a body and is operational. Further, generally, the implantable medical device used for monitoring includes transmifter/receiver circuitry for communicating information between the implanted device and a device external to the body, e.g., a programmer or external monitor.
For example, implantable monitoring devices, whether used solely as a monitoring device or in combination with other implantable therapeutic implantable devices, generally receive analog information from a sensor, store such information, and then transmit such information for usage external of the body. For example, the monitor may collect information regarding various physiological parameters of a patient such that a physician may scan records containing such information when the collected information is transmitted external to the body. The physician may then appropriately diagnose and treat the patient, e.g., assess changes in patient status, provide a therapy plan for the patient, recognize trends in such data, etc.
Generally, the most common method for storing and/or transmitting such sensor information is to first digitize the sensor information representative of one or more physiological parameters (i.e., change the analog signal to digital format) and then provide for storage of the digitized information in such a format. For example, as described in U.S. Pat. No. 5,535,752, a capacitive pressure sensing lead is employed with an implantable battery-powered monitor, including a microprocessor for implementing demodulation, data storage, and telemetry capabilities. The monitor samples and stores blood pressure data at programmed intervals and telemeters out the accumulated data to an external programmer on receipt of a command from an external device, such as in a manner which is conventional in implantable device technology. The monitor performs such periodic storage of digitized data related to physiological parameters, such as blood pressure and temperature, at a nominal sampling frequency which may be related to patient activity level. For example, such sampling frequency may be correlated to time and date and patient initiated event markers. As described in U.S. Pat. No. 5,535,752, blood pressure signals may be digitized and stored at a sample period of every 4 milliseconds or in other words, a 256 Hz sampling frequency. Further, for example, blood temperature signals may be digitized and stored once every sensed heart depolarization cycle. The digitized data values may be stored on a first-in first-out (FIFO) basis between periodic transmission of such data for permanent external storage outside of the device. External to the body, the data may then be analyzed to identify the portions of interest and to perform other diagnostic analysis of the accumulated data.
However, collecting and storing data for later communication to an external device, e.g., a programmer, at frequencies described above requires a large amount of memory to provide coverage for long periods of monitoring. For example, a typical storage period may be about 26 minutes for a single month of monitoring when the sampling frequency is approximately beat by beat, i.e., a sample taken every cardiac cycle. This becomes an undesirably large data set over the life of the implanted device.
Further, within such a data set there is a large variation in the data representative of the physiological parameter making it difficult to recognize a change or trend in any given physiological parameter. In other words, it is difficult to quantify and objectify the change within such large amounts of data.
In addition, such a large amount of data to be stored requires an undesirably large memory capacity, e.g., a large memory device, a large number of integrated circuits, etc. Such a large amount of memory is particularly undesirable for a small-sized implantable medical device and may cause excessive current drain from the battery during operation. Further, the large quantity of data collected and stored may require an undesirable amount of time to uplink such data to an external device. Although compression techniques are available, such compression techniques generally require extensive processing power which is typically not suitable for use in implantable medical devices.